1. Field of the Invention
The present invention relates to an electro luminescence device, and more particularly to an aging circuit for an organic electro luminescence device to prevent the deterioration of the electro luminescence device, and a driving method thereof.
2. Description of the Related Art
Recently, there has been developed various flat panel displays, which have the advantages of reduced weight and reduced bulk over a Cathode Ray Tube (CRT). Such flat panel displays include a Liquid Crystal Display (LCD), a Field Emission Display (FED), a Plasma Display Panel (PDP), and Electro Luminescence (hereinafter, EL) display device.
The structure and fabricating process of the PDP are relatively simple compared to the LCD, FED and EL devices. Another advantage of the PDP is that it can be made to have a large size but yet be light weigh. However, the light emission efficiency and brightness of a PDP is low while its power consumption is high.
Compared to a PDP, an LCD is difficult to make because of the semiconductor process for making the Thin Film Transistor (TFT), which is used as a switching device in each of the pixels in the LCD. The demand for LCDs has been increasing with the increasing demand of notebook computers because it is typically used as the display device of a notebook computer. However, the LCD has a disadvantage in that power consumption is high because the LCD uses a backlight unit. Further, the LCD also has the disadvantage of high light loss caused by the use of optical devices, such as a polarizing filter, a prism sheet, a diffusion panel. Another disadvantage of the LCD is a narrow viewing angle.
EL display devices are generally classified as either an inorganic EL device or an organic EL device depending on the material of a light-emission layer of the EL display device. Since an EL device is a self-luminous device, it has the advantages of a fast response speed, a high light-emission efficiency and high brightness. In addition, an EL device has the advantage of a wide viewing angle.
FIG. 1 is a sectional view representing an electro luminescence display device of the related art. As shown in FIG. 1, the organic EL display device includes a hole injection layer 3, a light emission layer 4, an electron injection layer 5 deposited between a cathode 6 and an anode 2 formed of a transparent electrode on a substrate 1. If a drive voltage is applied across the anode 2 and the cathode 6 in the organic EL display device, holes in the hole injection layer 3 and electrons in the electron injection layer 5 move into the light emission layer 4 and excite a fluorescent material within the light emission layer 4. Accordingly, a picture or an image is displayed by the visible light generated from the light emission layer 4 when a plurality of EL display devices are used together in an active matrix EL display panel.
In the organic EL device, a small-molecule organic EL material can be patterned by a vacuum deposition. In the alternative, a high polymer organic EL material can be patterned by a coating method using an inkjet spray head or a printing system. Construction of a high polymer organic EL will be explained in conjunction with FIG. 2.
FIG. 2 is a schematic plan view representing a pixel arrangement of an organic electro luminescence device of the related art. FIG. 3 is an equivalent circuit diagram of a pixel shown in FIG. 2. Referring to FIGS. 2 and 3, the organic electro luminescence device includes a number m of column lines CL1 to CLm, a number n of row lines RL1 to RLn to cross the column lines CL1 to CLm, and a number m×n of pixels P arranged in a matrix between the row lines and data lines.
Each pixel P of the organic electro luminescence device includes a first TFT T1 acting as a switching device formed at each intersection of the column lines CL1 to CLm and the row lines RL1 to RLn and a second TFT T2 formed between a cell drive voltage source VDD and an electro luminescence cell OLED for driving the electro luminescence cell OLED. The first and second TFT's T1 and T2 are p-type MOS-FETs. In addition, a capacitor is connected between the gate of the second TFT T2 and the cell drive voltage source VDD.
The first TFT T1 is turned on in response to a negative scan voltage from the row line RL1 to RLn. Thus a current path is enabled to conduct current between the source terminal and the drain terminal of the first TFT T1. Of course, the first TFT T1 remains in an “off” state when a voltage in the row line RL1 to RLn is below the threshold voltage Vth of TFT T1. A data voltage Vc1 from the column line CL is applied to the gate terminal of the second TFT T2 through the first TFT T1 during the on-time period of the first TFT T1. However, the current path between the source terminal and the drain terminal of the first TFT T1 is blocked during the off-time period of the first TFT T1 such that the data voltage Vc1 is not applied to the second TFT T2.
The second TFT T2 controls the current between the source terminal and the drain terminal in accordance to the data voltage Vc1 applied to its gate terminal. Accordingly, the electro luminescence cell OLED is made to emit light with a brightness corresponding to the data voltage Vc1. The capacitor Cst stores a voltage difference between the data voltage Vc1 and a cell drive voltage VDD to sustain the voltage applied to the gate terminal of the second TFT T2 for one frame period to uniformly sustain the current applied to the electro luminescence cell OLED for one frame period.
FIG. 4 is a waveform diagram representing signals applied to a column line and a row line shown in FIGS. 2 and 3. As shown in FIG. 4, the row lines are sequentially supplied with negative scan pulses SCAN and the column lines are simultaneously supplied with data voltages DATA that are synchronized with the scan pulses SCAN. While a scan pulse SCAN is applied to the gate of the first TFT T1, the data voltage DATA flows through the first TFT T1 to be charged in the capacitor Cst. In matrix array of such devices, the column lines CL are used to input picture signals, such as RGB data, to display a picture.
In the organic electro luminescence device as discussed the above, there is a disadvantage in that the switching performance of the switching transistors TFT T1 and TFT T2 deteriorates over time. In order to prevent such deterioration, an aging circuit is added to the organic electro luminescence device, the aging circuit applies an aging voltage in a reverse direction across transistors TFT T1 and TFT T2 for a set amount of time. In other words, the aging circuit applies voltages with polarities that are opposite to what is typically applied to the transistors TFT T1 and TFT T2.
FIG. 5 represents a pixel of an organic electro luminescence device to which an aging circuit is connected according to the related art. As shown in FIG. 5, the aging circuit 24 according to the related art is connected to the gate terminal and the drain terminal of the first TFT T1 of the pixel 22 of the organic electro luminescence device. The pixel area 22 of the organic electro luminescence device is configured in the same manner as described in FIG. 3, so the description of the pixel area 22 will be omitted with regard to the discussion of FIG. 5.
The aging circuit 24 includes a first aging switch device A1 connected between the first aging voltage source Va1 and the gate terminal of the first TFT T1, a second aging switch device A2 connected between a second aging voltage source Va2 and the source terminal of the first TFT T1, and a third aging voltage source Va3 to turn on the first and second aging switch devices A1 and A2. The aging circuit 24 applies an aging voltage to the electro luminescence cell OLED, wherein the final aging voltage is a drive voltage from the cell drive voltage source VDD. For this, the second TFT T2 must remain at on state while the aging is under way. For the second TFT T2 to be turned on, the second aging switch device A2 and the first TFT T1 must be on, and the first aging switch device A1 must be on for the first TFT T1 to be turned on.
Voltages Va1 and Va2, which are several times higher than the threshold voltages of the first and second TFT's T1 and T2, are sequentially applied to the gate terminals of the first and second TFT's T1 and T2, respectively. For example, if the electro luminescence cell OLED emits light with cell drive voltage source VDD of −15V and a ground voltage source GND of 0V, the third aging voltage source Va3 connected to the gate terminal thereof applies −30V such that the first and second aging switch devices A1 and A2 are turned on, the first aging voltage source Va1 through the first aging switch device A1 applies −25V to the gate terminal of the first TFT T1 such that TFT T1 is turned on, and the second aging voltage source Va2 applies −20V through the second aging switch device A2 and the first TFT T1 to the gate terminal of the second TFT T2 such that TFT T2 is turned on. Accordingly, while the aging process is under way for several minutes to several hours, since a high voltage is applied for a long time, the first and second TFT's T1 and T2 of the organic electro luminescence device deteriorate.